1. Field of the Invention
The present invention relates to a novel polymer material having charge transporting ability, which is applicable to various organic electro elements and devices utilizing the charge transporting function, such as photosensitive device for electrophotography, organic electroluminescent element, photorefractive element, electrochromic element, photosensor, solar battery or the like.
The present invention also relates to an organic electroluminescent element utilizing such novel polymer material having charge transporting ability, and further relates to a light emitting panel provided with such organic electroluminescent element.
2. Description of the Related Art
Charge transporting materials are materials having charge transporting ability. When charge is injected in the charge transporting material, the injected charge causes charge-concentration gradient or electromagnetic gradient within the material to lead diffusion or movement of charge, thereby exhibiting charge transporting activity. The term of “charge transporting material(s)” includes both meanings of an electron transporting material having ability of transporting electrons as charge and a hole transporting material having ability of transporting holes as charge. The charge transporting materials has been extensively investigated because it is important to produce organic electro elements and devices such as photosensitive device for electrophotography, organic electroluminescent element, photorefractive element, electrochromic element, photosensor, solar battery or the like.
For basic properties of the charge transporting materials, it is required: to be receptive to either one or both of electron and hole when it is in a neutral condition; to be high in the charge transporting ability; to be easily formed into a film-like form; and to be stable when it is formed into an amorphous film.
Since the charge transporting materials are often used as a uniform film, it is important to be easily formed into a film-like form. In a case where the charge transporting material is low molecular compound and formed into a film with 1 μm or less of thickness, vacuum deposition process is generally used as film-forming method. The vacuum deposition process however needs a depositing apparatus which is rather large-scale or strict than coating process to lead high cost, and use of the depositing apparatus also causes difficulty of producing a large substrate. In addition, when the low molecular compound is solely used to form a film, the film thus formed is inferior in mechanical strength or thermal stability. For these reasons, a coating technique is adapted in some cases, in which a film of the charge transporting material is formed by dispersing the low molecular compound in high molecular polymer as a binder, and coating with it.
On the other hand, most of the charge transporting materials are hole transporting materials, for which tertiary amine derivatives such as triarylamine or the like are extensively used. Electron transporting materials of applicable level include only a few sorts in comparison with the hole transporting materials because the electron transporting materials are not so high in solubility to an organic solvent and therefore not suitable to coating process for formation of film and also not so high in mobility of electron.
Among the charge transporting materials, there are those having bipolar property provided with both of hole and electron transporting functions. One of examples is CBP, namely 4,4′-Bis(Carbazol-9-yl)-biphenyl (Japanese Patent Application Laid-open No. Hei. 10-168443). In addition to the bipolar property, the CBP further has high transparency, and high matching to a light emitting dopant when the charge transporting material is used for the organic electroluminescent element. Therefore, the CBP is preferably used for an organic electroluminescent element using a phosphorescence dopant (M. A. Baldo et al., “Nature”, (2000), vol.403, p. 750).
However, the vacuum deposition process is still utilized as a major method to form the film or layer of the charge transporting materials such as the CBP. Moreover, since molecules of the CBP tend to form a plane structure, an amorphous film of the CBP is easily crystallized by time passing or heat. Therefore, in a case of using the CBP for an electronic device which is attended with heating generation of the Joule heat, such as the organic electroluminescent element, there is a defect that large amount (about 5 to 10 percent) of doping is required or that mixing of the binder is required for prevention of crystallization. For these reasons, it is difficult to form the film or layer by solely coating with the CBP.
Turning the attention to electroluminescent elements (hereinafter, “electroluminescent” will be simply referred as “EL”.), the EL elements utilize light emission in the electric field, and have several advantageous characteristics of being adaptable to a thin and compact design, having a high visibility and a wide angle of visibility because of self-light emission, being good in impact resistance because it is really in the solid state. Thus the EL elements are expected to be utilized as a light emitting element of various displaying device. Among the EL elements, there are inorganic EL elements using inorganic compound as light emitting material and organic EL elements using organic compound as light emitting material.
In particular, the organic EL elements can remarkably lower the voltage to be impressed as well as having easiness of size-reduction, whereby enabling reduction of the electric power consumption. In addition, it enables light emission from plane and light emission of three primary colors. Thus the organic EL elements are subjected to investigation for developing a next generation of light emitting element.
For a layered structure of the organic EL elements, a fundamental structure is “a positive electrode/an organic light emitting layer/a negative electrode”, and there are known other structures which are further provided with a hole injection transporting layer and/or an electron injection layer at a proper position, such as “a positive electrode/a hole transporting layer/an organic light emitting layer/a negative electrode”, and “a positive electrode/a hole transporting layer/an organic light emitting layer/an electron injection layer/a negative electrode”.
The conventional organic EL elements utilize light emission taken from fluorescent material. The organic EL elements take energy in an excited state as light emission, the excited state being caused by recombination of electrons and holes injected in the element. The excited state caused is considered to include 25% of singlet state and 75% of triplet state. In the organic EL elements utilizing fluorescence, only the energy of singlet state is utilized, and therefore inner quantum efficiency is restricted within 25% according to the principle. Attentions are now imposed on organic EL elements utilizing phosphorescence. The phosphorescent organic EL elements can utilize the energy of triplet state as well as that of singlet state, and it enables the inner quantum efficiency to rise up to 100% according to the principle. In the phosphorescent organic EL element, a light emitting material comprising metallic complex containing heavy metal such as platinum, iridium or the like is used as a dopant emitting phosphorescence, and a host material is doped with such light emitting material to take the phosphorescent emission (M. A. Baldo et al., “Nature”, (1998), vol.395, p.151; M. A. Baldo et al., “Apllied Physics Letters”, (1999), vol.75, p.4; M. A. Baldo et al., “Nature”, (2000), vol.403, p.750).
Light emission by the phosphorescent dopant has relativity to the host material. Basic properties required to the host material include: a hole or electron transporting ability; a reduction potential of the host material higher than that of the phosphorescent dopant; an energy level of triplet state of the host material lower than a reduction potential of the dopant; or the like, and CBP, namely 4,4′-Bis(Carbazol-9-yl)-biphenyl (Japanese Patent Application Laid-open No. Hei 10-168443), is preferably used in general.
However, charge transporting materials such as the CBP which are used as the host material for the organic EL element utilizing the phosphorescence emitting material is liable to crystallize, and there are defects that a large amount (about 5 to 10 percent) of doping for prevention of crystallization, or that the film- or layer-formation through the coating process is difficult. Therefore, it is difficult to produce the organic EL element having high efficiency of light emission by the coating process using the CBP.